1. Field of the Invention
The present invention relates to a protective, diffusive film and more particularly to a high-quality protective, diffusive film which can suppress curling and deflection, a process for producing the same, and a surface light source device and a liquid crystal display device using said protective, diffusive film.
2. Background Art
A large number of surface light source devices provided with a protective, diffusive film have hitherto been proposed. An example of a surface light source device using a protective, diffusive film 110 is shown in FIG. 5. FIG. 5 is a cross-sectional view of a liquid crystal display device 135 provided with a surface light source device 120 which is an edge-type flat light source. The surface light source device 120 comprises, for example, a light source 121, a light guide plate 122, a reflection film 124, a light diffusive film 125, a lens film 140, and a protective, diffusive film 110.
The light guide plate 122 is surface light projection means, and has a light source 121 on its side end and, on its non-light-outgoing surface remote from a light outgoing surface 122a, a dot pattern 123 for diffusing light from the light source 121 and directing the light to a light outgoing direction. The reflection film 124 is provided on the non-light-outgoing surface side of the light guide plate 122 and functions to block off unnecessary light beams and, at the same time, to reflect and return light beams to a predetermined direction.
A lens film 140 is disposed on the light outgoing surface 122a side of the light guide plate 122 through a light diffusive film (a diffusive plate) 125 for diffusing light to conceal the dot pattern 123 in such a manner that the lens film 140 on its prism surface 140a faces the light outgoing surface side.
In order to impart the function of diffusing light to the light diffusive film (diffusive plate) 125, the light diffusive film (diffusive plate) 125 has hitherto been formed by incorporating, in a dispersed state, organic or inorganic beads as a light diffusing agent into a transparent resin substrate, or by coating an ink containing organic or inorganic beads as a diffusing agent onto a transparent resin substrate.
A protective, diffusive film 110 is provided on the light outgoing surface side of the lens film 140, for preventing the prism 140a in the lens film 140 and a liquid crystal display element 133 from coming into direct contact and being damaged by each other, for example, due to vibration in transit. The protective, diffusive film 110 also has the function of slightly diffusing light for concealing stripes of the prism 140a in the lens film 140, a spacer (not shown), and the like. To this end, the protective, diffusive film 110 has hitherto been formed by incorporating, in a dispersed state, organic or inorganic beads as a light diffusing agent into a transparent resin substrate, or by coating an ink containing organic or inorganic beads as a diffusing agent onto a transparent resin substrate. A transmission liquid crystal display element 133 comprising a liquid crystal layer 130 sandwiched between a lower substrate 132 and an upper substrate 131 is provided on the light outgoing side of the surface light source device 120, and the liquid crystal display element 133 is illuminated from backside by the surface light source device 120.
The conventional protective, diffusive film 110 is generally formed by coating an diffusing ink (an ink prepared by adding a diffusing agent in a resin) on both sides of a transparent substrate. As shown in Table 1, there is a difference in coverage between the coating on the top surface of the transparent substrate and the coating on the under surface of the transparent substrate.
In the conventional protective, diffusive film, when the difference in coverage between the coating on the top surface of the transparent resin substrate and the coating on the under surface of the transparent resin substrate is large, curling and/or deflection are likely to occur through the action of heat and/or humidity. This is considered attributable to the difference in stretching, derived from the difference in coverage, between the diffusing ink coating on the top surface of the transparent resin substrate and the diffusing ink coating on the under surface of the transparent resin substrate. In particular, under conditions within backlight for a liquid crystal display device which is brought to a closed space in which the temperature is raised to a high temperature, i.e., up to about 90xc2x0 C., xe2x80x9cdeflectionxe2x80x9d of the film sometimes occurred. When the coverage of the coating on the light source side (under surface) was made substantially equal to the coverage of the coating on the viewer side (top surface), the following phenomena often occurred. (i) The concaves and convexes on the surface were reduced, and interference fringe appearance defects between the protective, diffusive film and the underlying light guide plate or lens film sometimes occurred. (ii) Increasing the amount of the diffusing agent (fine particles) enhanced diffusive properties (i.e., enhanced the haze value) and consequently lowered the utilization of light and sometimes deteriorated luminance.
In the conventional protective, diffusive film, the diffusive properties was imparted by adding a diffusing agent to the resin. Therefore, a variation in coverage caused a change in diffusive properties or a change in the shape of surface concaves and convexes. This sometimes affected the optical characteristics and the sticking preventive properties.
The present inventor has now found that, when resin layers are provided respectively on the top surface and the under surface of the substrate layer by not using any diffusing agent (for example, finely particulate material) and bringing the difference in coverage between the resin layer provided on the top surface of the substrate layer and the resin layer provided on the under surface of the substrate layer to a value falling within a specific range, curling and deflection can be effectively prevented and, consequently, a high-quality protective, diffusive film can be provided. The present invention has been made based on such finding.
Accordingly, it is an object of the present invention to provide a protective, diffusive film possessing excellent optical characteristics and sticking preventive properties.
According to one aspect of the present invention, there is provided a protective, diffusive film provided with a lens film, said protective, diffusive film comprising:
a transparent substrate layer; and
a resin layer provided on the top surface and the under surface of the transparent substrate layer,
said resin layers being free from fine particles and having fine concaves and convexes on the surface thereof,
the difference in coverage between the resin layer provided on the top surface of the transparent substrate layer and the resin layer provided on the under surface of the transparent substrate layer being in the range of xe2x88x9220% to +20% in terms of the percentage of the coverage of one of the resin layers to the coverage of the other resin layer.
According to another aspect of the present invention, there is provided a process for producing the protective, diffusive film according to the present invention, said process comprising the steps of:
introducing an ionizing radiation-curable resin into a cylinder having an inverted shape of concaves and convexes of the resin layer; and
applying an ionizing radiation to the ionizing radiation-curable resin to cure the ionizing radiation-curable resin to form concaves and convexes.